Circulating Sub/Swage A circulating sub falls under the downhole accessory tool category that regulates flow rates, especially drilling in slimhole wells or wellbore cleanout projects. Circulating sub is manufactured to the following specs connections, lengths, weights and material grade. Crossover Circulating Sub/Swages are manufactured from AISI 4140/4145. They typically are female hammer union on the top end and an IF pin on the bottom end, but can be manufactured with any size and style connection.